The present invention relates to a method of producing a porous material having open pores. More particularly, the invention is concerned with a method of producing a porous material having open pores from a raw material which comprises a bisphenol-type epoxy resin, a specific mixture amide compound as a hardener, filler and water.
Hitherto, as measures for producing a porous material having open pores for use as a filtering medium, air diffusion medium, casting mold and so forth, various methods have been produced such as sintering of metal powder, sintering of powdered thermoplastic resin, sintering of inorganic powder, hydration setting of cement or the like, pressing or stamping of a mixture of thermosetting resin and filler, hardening of a resin liquid containing pore-forming agent followed by the removal of the pore-forming agent by dissolving, extraction or evaporation, use of a blowing agent, evaporation of water from water-containing polyester resin, and so forth.
These known methods of producing porous material having open pores, however, encountered one or more of the following problems in connection with the molding process. First of all, it is to be pointed out that these known methods impractically limit or restrict the shape and size of the product. In addition, these methods often require a heat treatment at high temperature, as well as a press work at high pressures. The method which makes use of the pore-forming agent must employ a step of heating or vacuum operation for the removal of the pore-forming agent by evaporation. Furthermore, these known methods are generally complicated and difficult to conduct.
Another problem is that, with these known methods, it is quite difficult to effect the pore size control which is the most important factor when the porous material is used as a filtering medium, air diffusion medium, casting mold or the like due to the following reasons.
In the production of a porous material from metal powder by sintering, it is difficult to obtain a pore size smaller than 5 microns because of a specific relation between the particle size of the metal powder and the surface energy during the sintering. Consequently, it is quite difficult to effect control of the pore size to obtain pores on the order of 1 micron or so. In the case of the sintering of the powder of a ceramic material or resin, the packing density at the time of molding is almost impossible to control although the particle size of the material can be controlled finely. In addition, this method suffers from a large change in the pore structure in the course of the sintering, so that the pore size is distributed over a wide range to hinder the control of the pore size.
Referring now to the pressing and stamping of a solid powder together with a binder such as an adhesive, the control of the pore size is impaired particularly when the product has a complicated shape, for the reasons such as non-uniform packing due to non-uniform application of the molding pressure attributable to the complicated shape of the product, non-uniform mixing of the binder, and so on.
In the case of the hydration hardening of a cement, gypsum, plaster or the like, the difficulty is involved in the control of the nucleation and growth of the crystals of the hydrate, and precise control of the mean pore size to obtain pores on the order of 0.5 to 10 microns can hardly be effected.
With regard to the method in which the pore-forming agent is evaporated or extracted from the hardened resin containing the agent, a technique has been proposed for the production of a thin film having pores of pore sizes ranging between 1000 and 100 .ANG.. This method, however, cannot be applied to the production of a product having a considerably large shape. In the method in which a resin in the form of O/W- or W/O-type emulsion is hardened followed by the evaporation of the dispersoid, it is not easy to control the size of the dispersoid, and many disconnected pores are inevitably formed. The control of the pore size on the order of 0.5 to 10 microns as a mean is also difficult in this method.
In order to obviate these problems of the known methods, Japanese Patent Publication No. 2464/1978 of the same applicant proposes a method of producing a porous material having open pores and a complicated shape, at a high precision and with a good control of the pore size. More specifically, this method comprises the steps of preparing an O/W type slurry as a mixture of a glycidyl-type epoxy resin, polymeric fatty acid polyamide hardener, filler and water, casting the slurry in a water-impermeable mold, hardening the water-containing slurry, and then dehydrating the hardened body. According to this method, it is possible to effect the control of the pore size by varying and adjusting factors such as the particle size of the filler, amount of the reactive diluent, mixing ratio of the epoxy resin, hardener, filler and water, and so on. Among these factors, the amount of the reactive diluent and the mixing ratio of the epoxy resin, hardener, filler and water are limited from the viewpoint of contraction during hardening and also in view of the strength of the hardened body. For this reason, it is not possible to control the pore size over a wide range. In order to control the pore size to obtain pores on the order of 0.5 to 10 microns, it is necessary to largely vary the particle size of the filler. The use of the filler having large particle size, however, reduces the strength of the hardened body undesirably. The reduction of strength can be avoided only through a reduction in the amount of water added, which in turn is accompanied by a new problem of an increase in the viscosity of the slurry. The increased particle size of the filler causes also another problem namely that the efficiency of the work is impaired due to sedimentation of the filler in the slurry which consists of water and the filler. When a filler having a mean size suitable for the work is used, the mean pore size in the product porous material is undesirably concentrated to an extremely small level of less than 1.5 microns. The porous material having such extremely small pore size can hardly be used for the aforementioned applications such as filtering medium, air diffusion method, casting mold and so forth and, hence, has only a small practical utility. Furthermore, this proposed method suffers from problems such as the impractically long time required for the slurry to flow into the mold and difficulty in forming a porous material having an intricate and complicated shape, due to the high viscosity of the slurry attributable to the use of a polymeric fatty acid polyamide solely as the hardener.